A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. AC-DC power converters convert an alternating current (“ac”) input voltage into a direct current (“dc”) output voltage. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of power switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
To produce a dc output voltage, power converters often employ diodes to rectify an ac voltage. The rectifying devices can introduce a power loss component in a power converter due to the forward voltage drop across the diode, particularly in a power converter that produces an output voltage of five volts or less. Schottky diodes, which have a relatively low forward voltage drop, are often employed in low-voltage power converter applications to reduce a diode forward voltage drop. However, passive rectifying devices such as Schottky diodes typically cannot achieve forward voltage drops of less than about 0.35 volts (“V”), and typically cannot sustain a reverse voltage greater than about 60 volts, thereby limiting a conversion efficiency of the power converter.
To achieve an acceptable level of efficiency, power converters often employ rectifying devices that may have forward voltage drops less than about 0.1 volts. To provide such reduction of power loss, an active switch or active semiconductor switch such as a metal-oxide semiconductor field-effect transistor (“MOSFET”), which provides a resistive voltage drop, is often employed to replace a diode. An active semiconductor switch, however, must be periodically driven into conduction and non-conduction modes or states in synchronism with a periodic waveform of an ac voltage (e.g., an ac voltage produced across an input to the power converter). The active semiconductor switches may thereby avoid the higher forward voltage drops inherent in the passive rectifying devices.
A design issue introduced by substituting an active semiconductor switch for a diode is the need to provide a drive signal therefor that is accurately synchronized with the operation of the power converter to control the conduction and non-conduction modes or states of the active semiconductor switches, and that avoids conduction overlap with other semiconductor switches including diodes. An active semiconductor switch substituted for a diode in a power converter is generally referred to as a “synchronous rectifier” or “synchronous rectifier switch.”
A conventional ac-to-dc power converter employs a bridge rectifier to transform an ac sinusoidal input voltage waveform, such as an input voltage waveform produced by an ac mains, into a rectified sinusoidal waveform. Following the bridge rectifier, a power factor correction (“PFC”) circuit converts the rectified sinusoidal waveform into a dc waveform with a dc voltage level higher than the peak voltage of the sinusoidal input voltage. The bridge rectifier is usually constructed with four diodes. Due to the forward voltage drop of the diodes, significant power losses are produced by the diodes.
A bridgeless boost PFC circuit may be employed to address the power loss problem associated with the forward voltage drop of the diodes. As a result, conduction losses of the diodes are reduced. However, a bridgeless boost PFC circuit has several significant remaining problems that limit its application in low-cost, high-volume circuits. On the ac side of the circuit, a two-inductor structure introduced by the circuit causes the output voltage to float with respect to the input line voltage. As a result, the circuit produces a high level of electromagnetic interference (“EMI”) noise. The location of the boost inductor on the ac side makes it difficult to sense the ac line voltage and the inductor current and the control is a more complicated process compared with control schemes used for a simpler boost PFC circuit or boost power converter coupled to a bridge rectifier. As a result, the bridgeless boost PFC circuit has found limited application.
U.S. Pat. No. 6,060,943, entitled “Circuit Simulating a Diode” to Jansen, issued May 9, 2000 and U.S. Pat. No. 6,469,564, entitled “Circuit Simulating a Diode” to Jansen, issued Oct. 22, 2002, which are both incorporated herein by reference, are directed to a circuit that performs the function of a diode to conduct current in one direction with a low forward voltage drop, but block current in the other direction to produce an improved rectification function. When the voltage at a designated anode terminal of the circuit is higher than the voltage at a designated cathode terminal, a forward current flows. When the polarity of the voltage at these designated terminals is reversed, the current is interrupted.
Each of these approaches, however, provides an efficiency and/or a cost limitation that limits or otherwise penalizes the use of a synchronous rectifier in many applications. Accordingly, what is needed in the art is a controller employable with a synchronous rectifier in a power converter and related method that avoid the deficiencies in the prior art.